<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    .red{
        background-color: red;
    }
    .green{
        background-color: green;
    }
    .size{
        width: 300px;
        height: 300px;
    }

</style>
<body>
    <div class="red size"></div>
    <div class="green size"></div>
    <div class="red size"></div>
</body>
</html>